This seems the most sensible, presuming the originating package is OK with it. In general, might be nice to have a description as to why extending a function is discouraged. My impression is that it reduces maintainability (and might lead to a proliferation of conflicting definitions if eg the original package implements a function that does the same thing but in a different way).
If the package won’t accept a PR, that’s another thing entirely, but I get the impression that that sort of behavior is rare in this community.